1. Field of the Invention
The present invention relates to cooling methods and systems that employ refrigerants in their supercritical state combined with microchannel cooling technology in closed or open cooling cycles.
2. Prior Art
Reliable and effective cooling methods and systems are one of the basic problems in development of future electronic equipment, jet and internal-combustion engines, spacecraft systems and structures, fuel cell technologies, infrared radiation detectors, nuclear power generation, and others.
Cooling electronic systems is the one of the most acute problems. Modern electronic devices operate at higher power densities and higher temperatures than before and create more waste heat. Current and future electronic devices can produce heat fluxes of more than 1,000 W/cm2. These heat fluxes will raise device temperatures and lead to reduced efficiency and lower durability. The rapidly growing computer industry is continuously in search of new ways to cool microprocessors. Innovative chip architectures create localized hot spots which require the development of on-demand, locally addressable cooling methods and solutions.
The abundance of worldwide patent documents for cooling methods and device applications highlights the importance of the cooling problem. It also suggests that approaches and solutions previously proposed for a variety of cooling methods and sophisticated cooling systems could not entirely guarantee the reliable and extended service life of critical components having high temperature thermal emissions.
It is well known that in order to reliable handle large thermal loads that contribute to thermal resistance, it is essential to maximize and/or optimize the heat exchange surface contact area between the cooling agent (refrigerant) and the cooling head itself. If the heat exchange area is not adequate for the heat load, the critical electronic device becomes thermally “decoupled” from the refrigerant and may be significantly overheated. A substantial increase in heat exchange area can be achieved by streaming a refrigerant through a network of microchannels. Thermally conductive materials structured with microchannels less than 50 μm in diameter are known to have a surface area of about (1 . . . 10) m2/g, or even more with a high thermal capacity for removing a large quantity of heat. Microchannel cooling thus has distinct advantages over any other cooling method.
Currently the most efficient cooling method using microchannel technology is based on the two-phase flow regime of liquid refrigerants that are in thermal contact with the objects to be cooled. Conventionally, the microchannel cooling is achieved inside a microchannel heat sink thermally connected with the surface containing the electronic device. This microchannel heat sink is manufactured from a solid substrate having a high thermal conductivity. It contains the microchannels of varied complex configurations that serve as flow passages for the liquid refrigerants and that increase the surface contact area between the liquid refrigerant and the electronic device. The heat generated by the electronic device is transferred to the microchannel heat sink and removed from the heat sink by the evaporation of the liquid refrigerant flowing through the microchannels.
Two-phase liquid cooling using the latent heat of evaporation of the liquid refrigerant is capable of absorbing a large amount of the waste heat, but the biggest downside of a two-phase liquid cooling system is the potential for rapid failure due to a condition known as “vapor lock”. For example, as the electronic device with high heat emission begins to heat, the liquid refrigerant in thermal contact with the electronic device begins to boil and converts to vapor. Evaporation causes an enormous expansion of volume (i.e., 1:1,000, or even more) resulting in exceptionally poor gas flow cooling compared to that achieved within the liquid phase. The small volume of a microchannel cooling system excludes an efficient application of two-phase liquid cooling since the liquid refrigerants cannot continue to flow against the massive volume of the gas ahead of itself.
All prior attempts at two-phase liquid flow cooling accompanied by evaporation required sophisticated designs to avoid “vapor lock”. However, the efficiency of the heat transfer inevitably will be limited by the existence of Critical Heat Flux (CHF) above which the two-phase cooling system suffers a “boiling crisis”. The “boiling crisis” appears when the vapor bubbles on the heated surface abruptly form a thin film that thermally insulates the now dry hot surface from the cooling liquid. In this case heat transfer to the fluid will be blocked and the temperature of the object to be cooled will increase rapidly leading to the complete destruction of the electronic device. Engineering solutions to avoid the “boiling crisis” practically exclude the application of two-phase liquid cooling in future electronic device architectures.
The new cooling methods and systems claimed by the present invention relate to the application of refrigerants maintained at their supercritical fluid state to prevent the “vapor lock” and “boiling crisis”. The patent materials below are the closest to the present invention.
U.S. Pat. No. 4,205,532 by Brenan discloses a heat pump that comprises a closed circuit containing an acceptor (evaporator) for heat absorption by a refrigerant, a compressor for compressing the heated refrigerant issued from acceptor, a rejector (condenser) for heat rejection by the compressed refrigerant, and an expansion device to expand the refrigerant from the rejector before it is directed back to the acceptor. The acceptor and rejecter are the counter-current heat exchangers, and the compressor compresses the refrigerant leaving the acceptor to reach its supercritical pressure while simultaneously raising its temperature. The compressed refrigerant is directed into the heat rejector that decreases its temperature without pressure change, and the expansion device expands the refrigerant leaving the rejector thereby reducing further its temperature and simultaneously its pressure.
The heat pump embodiment of this invention is constructed along the same lines as a conventional heat pump where refrigerant being compressed to reach its supercritical pressure is directed into the heat rejecter (condenser). The expansion device must provide a sufficient degree of throttling to reduce refrigerant pressure to a suitable subcritical value before it enters the acceptor (evaporator) to absorb heat. The disclosed heat pump can be employed to heat a fluid to a temperature in excess of the critical temperature of a refrigerant. Since the refrigerant in the rejector (condenser) is under supercritical pressure corresponding to a single-phase fluid, there is no requirement for liquid drainage through the rejector.
The same vapor compression cycle is used in refrigeration, air-conditioning and heat pump systems operating under supercritical conditions that are disclosed in U.S. Pat. No. 5,245,836 by Lorentzen et al., U.S. Pat. No. 5,405,533 by Hazlebeck et al., U.S. Pat. No. 6,105,380 by Yokomachi et al., U.S. Pat. No. 6,343,486 by Mizukami, U.S. Pat. No. 6,591,618 by Howard et al., U.S. Pat. No. 6,658,888 by Manohar et al., U.S. Pat. No. 6,698,214 by Chordia, U.S. Pat. No. 6,848,268 by Memory et al., and in U.S. Pat. Applications Nos. 2003/0102113 by Memory et al., 2006/0059945 by Chordia et al., 2006/0086110 and 2007/0000281 both by Manole, 2006/0123827 by Achaichia, and WO 2007022778 by Christensen.
The conventional vapor compression process producing supercritical fluids which further reject heat absorbed by the evaporation of a liquid refrigerant under subcritical conditions is the basis of the most prior patent documents. This significantly limits the difference between the enthalpy at the entrance of the rejector and the enthalpy at the exit of the rejector due to the limited temperature interval of the supercritical fluids to be used. This shortcoming is common for all cited prior patent documents based on the application of supercritical fluids. Moreover, the use of supercritical fluids to reject heat significantly reduces the efficiency in the cooling cycle because the heat absorption is provided by a conventional two-phase evaporative liquid. The two-phase evaporative liquid will unavoidable limit the effectiveness of microchannel cooling because the unique thermodynamic properties of the supercritical fluids will be ignored and the problem of “vapor lock” will remain unresolved.
The attempts to overcome “vapor lock” are disclosed in U.S. Pat. No. 6,024,542 and U.S. Pat. No. 6,179,568 both by Phillips et al., U.S. Pat. No. 6,967,840 by Chrysler et al., U.S. Pat. No. 7,035,104 by Meyer as well as in U.S. Pat. Application No. 2003/0062619 by Ritz et al.
The '542 and '568 patents describe sophisticated piston pumps and the method of reducing “vapor lock” by mixing the vapor and liquid portions of a refrigerant and introducing the mixture into the piston pump. The '840 patent discloses the clearing of “vapor lock” in a microchannel cooling subsystem. A pumping mechanism and auxiliary flow generator, both coupled to an input of the microchannel cooling subsystem, provide two different pressure levels to create turbulence in the fluid flow that contributes to the clearing of “vapor lock” inside the microchannels. The '104 patent describes a mini/microchannel multi-level-cooling enhancement stud of the cooling system that is thermally connected with a heat dissipating device mounted to the liquid coolant module containing subcooled liquid coolant. Heat removal from such a device is accompanied by routine intensive boiling of the liquid coolant, and vapor bubbles nucleate, grow, and depart at high frequency, constantly being replaced by incoming jet-impinged subcooled liquid coolant.
All of the proposed cooling systems may delay “vapor lock” in some instances, but they cannot completely resolve this problem because of unavoidable technical and thermodynamic difficulties that practically limit the application of all two-phase cooling technologies.
The '619 patent discloses a heat transfer system to maintain the fluid in its supracritical state without any changes in phase of the fluid thus avoiding “vapor lock”. A thermoelectric cooler is used to remove heat from the supracritical fluid that is in thermal contact with a heat-generating surface for a sufficiently short dwell time. A single-phase supracritical carbon dioxide operating at critical pressure and temperature interval between 25° C. and 40° C. has the same disadvantages described above due to a limited difference between the enthalpy in the start and end points of the cooling process.
Accordingly, it is a principal object of the present invention to form novel cooling methods and systems using supercritical fluids that are capable of absorbing a large quantity of the heat within microchannel cooling devices using closed or open cooling cycles and avoiding “vapor lock” inside the microchannels.